I have removed the stock exhaust on my 89 suburban TBI. I added long tube headers and installed a y pipe back in to the system. High performance cat with a flowmaster muffler in the system. From the back of the headers its 2.5 tubes going in to the y and goes 2 into 1 with a 3" pipe for cat and muffler. After muffler it splits back up to 2 2.5 pipes out the back.

Problem now is that it runs better but when I push the gas and let of quickly I have a small whistle coming from the engine compartment. It only does it when I let off the gas quickly. I just got a vacum gauge going to see what I kind find out. I "think" it may be coming from the front or rear seal on the intake. Any ideas?